This invention relates generally to spindle motors for use in information, audio and video equipment and more particularly to a fluid bearing motor fit for use in a magnetic disk unit, an optical disk unit or the like.
As a conventional fluid bearing motor, there is a fluid bearing motor for a magnetic disk as shown in FIG. 9, for example.
In this fluid bearing motor, a thrust collar 51 is fixed to the outer peripheral surface 50a of a shaft member 50, and a thrust fluid bearing portion 100 is formed among the outer peripheral surface 50a, an undersurface 51b of the thrust collar 51 and an upper surface 52b of a sleeve 52 which faces the undersurface 51b.
Further, a radial fluid bearing portion 200 is formed under the thrust collar 51 and between the outer peripheral surface 50a of the shaft member 50 and an inner peripheral surface 52c of the sleeve 52.
In FIG. 9, reference numeral 53 denotes a stator, 54 indicates a rotor magnet, and 55 defines a back yoke.
However, bubbles may remain in the thrust fluid bearing portion 100 of such a fluid bearing motor when a lubricating fluid is poured into the thrust fluid bearing portion 100 and may also be mixed with the lubricating fluid in the thrust fluid bearing portion 200 due to the repetition of starting and stopping the operation. When the temperature of the lubricating fluid rises in such a state that the bubbles have thus been mixed therein, the lubricating fluid may be forced out through the bearing gaps because of the expansion of air, to thereby occur a problem that the reliability of the fluid bearing motor is deteriorated.
Moreover, the lubricating fluid tends to scatter because of the high-speed rotation or otherwise to become easily exhausted from the thrust fluid bearing portion as it evaporates during long-term use. Another problem is that the thrust fluid bearing may often seize during long-term use.
Method for solving the foregoing problems is suggested in the Japanese Patent Unexamined Publication No. Hei. 8-163820. The invention taught by the publication No. Hei. 8-163820 is such that, as shown in FIG. 10, a groove 60 is formed circumferentially in the outer peripheral face of a thrust collar 51 in such a way as to form a true circular contour 60a which is eccentric with respect to the thrust collar 51 as well as a shaft member 50 in order to make the shallow portion of the groove 60 or a portion without the groove 60 a reservoir 61 for storing a lubricating fluid while making the deep portion of the groove 60 an air chamber 62 by varying the volume of the diametric space between the thrust collar 51 and the inner peripheral face of a sleeve 52. Further, providing a fluid channel 63 for communicating the air chamber 62 with the atmosphere causes the bubbles mixed with the lubricating fluid in the bearing gap to be gathered in the air chamber 62 before being discharged outside through the fluid channel 63.
Further, an oil supply channel 64 similar in structure to the fluid channel 6.3 communicates with the reservoir position opposite to the air chamber 62 (if the groove 60 is shallow or otherwise non-existent). In this conventional fluid bearing motor, it is intended to prevent the lubricating fluid from being exhausted from the thrust fluid bearing portion by supplying the lubricating fluid from the reservoir 61 via the oil supply channel 64 to the inner diameter side of the thrust fluid bearing portion.
The contour of the outer peripheral face of the thrust collar 51 and the contour of the inner peripheral face 52a of the sleeve 52 diametrically opposite to the outer peripheral face thereto as disclosed in the Japanese Patent Unexamined Publication No. Hei. 8-163820 are true circles in both cases.
Although the oil supply channel 64 is allowed to communicate with the reservoir 61 in the device of the Japanese Patent Unexamined Publication No. Hei. 8-163820, the present inventors found by experiment that the existence of the oil supply channel 64 resulted in weakening the generation of a wedge film itself and instead of supplying the lubricating fluid from the reservoir 61 onto the inner diameter side of the thrust fluid bearing portion the oil supply chambers causes bubbles to be readily discharged into the lubricating fluid bearing within the reservoir as the air flowed backward under the influence of the oil supply channel 64 which was opened to an area where the negative pressure passing the pressure peak of the wedge film is generated, that is, opened close to the area where the cavitation occurs. Since the bubbles thus discharged revolve in the eccentric groove 60 formed in the outer peripheral face of the thrust collar as the fluid revolves, there develops a problem of slightly moving the rotational axis during the revolution of the bubbles.
It was also found from the results of observation above that in such an arrangement as to provide the reservoir and the air chamber by forming the true circle eccentrically with reference to the rotational center, the rotational axis was slightly moved. In other words, the rotational center was forced to move in one direction by the pressure generated by the wedge effect of the reservoir portion having only the narrow gap as the speed increased even though no bubbles were generated; the problem in this case is that an unfavorable influence resulting therefrom becomes conspicuous during the high-speed rotational operation in particular.
When applied to magnetic disk units, fluid bearing motors are required to have higher rotational accuracy with the progress of developing-high-speed disk units and attaining improved surface recording density. Consequently, even slight movement of the rotational axis during the high-speed rotational operation becomes increasingly problematical and in addition to improvement in reliability and durability, it is urgently necessary to solve the foregoing problems.